The present invention relates to an air-fuel ratio control system with an electrically-controlled carburetor in an intake system of an internal combustion engine and with a three-way catalytic converter in an exhaust system for detecting the oxygen concentration of exhaust gases. The air-fuel ratio control system controls the air-fuel ratio of an air-fuel mixture to a stoichiometric air-fuel ratio at which the three-way converter is most effective and controls the air-fuel ratio to a fixed rich air-fuel ratio (a small air-fuel ratio) during middle- or high-speed driving and during heavy load driving, so that the controlled air-fuel ratio range and the fixed air-fuel ratio range are adjusted properly to the vehicle driving conditions.
In Japanese patent application, Nos. 54-98921 and 55-10678, the present applicant has already proposed an air-fuel control system provided with a detecting device for detecting a heavy load operation of the engine and with a control circuit for controlling the air-fuel ratio to a fixed rich ratio in order to produce high power for the heavy load operation.
In such a conventional system, because the intake passage vacuum and the throttle opening degree vary in dependency on the load, a certain value of the intake passage vacuum or throttle opening degree is selected for the detection of the heavy load operation. For instance, in a load characteristic shown in FIG. 1, a curve A is a torque curve of the full throttle open condition and a curve B shows a detecting characteristic of a vacuum detecting device which operates at -80 mmHg. By the intake passage vacuum detecting method, in the range lower than the curve B, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio, but in the range higher than the curve B the ratio is fixed to a rich air-fuel ratio for powerful operation. Also in the throttle opening degree method, ranges are defined by a curve C.
As clearly shown in FIG. 1, the fixed air-fuel ratio range higher than the curve B exists in the low engine speed zone with a considerable width and is narrow in the high speed zone. Such a low speed driving is usual mainly on urban streets. However, the powerful operation by the fixed air-fuel ratio is not necessary on urban streets, but it is important to decrease noxious exhaust gas emission. In addition, it is unfavorable under high speed driving under heavy load conditions, for the fixed air-fuel ratio range to be narrow in a high speed zone.
On the other hand, by the throttle opening degree detecting method the fixed air-fuel ratio range at the low speed zone is narrower than with the vacuum detection method, but the exhaust gas purification effect is still insufficient. Further, the fixed air-fuel ratio range at the middle and high speed zone is broad, resulting in increases of noxious gas emission.